1. Field of the Invention
The present invention generally relates to the deposition of a metal film on a substrate. More particularly, the present invention relates to positioning of a substrate holder used during deposition of a metal film on a substrate.
2. Background of the Related Art
Electroplating, previously limited to the fabrication of lines on integrated circuit boards, is now used to deposit metals on seed layers formed on substrates. The multiple layers deposited on a substrate during electroplating include a diffusion barrier layer, a seed layer, and a metal film. The metal film fills a feature formed in the seed layer on the substrate. One embodiment of a feature-fill process that involves electroplating comprises initially depositing the diffusion barrier layer over surfaces of the substrate that are to be electroplated by a process such as physical vapor deposition (PVD) or chemical vapor deposition (CVD). Next, a seed layer is deposited over the diffusion barrier layer by a process such as PVD or CVD. The metal film is then deposited over the seed layer on the substrate by electroplating. Finally, the deposited metal film can be planarized by another process such as chemical mechanical polishing (CMP), to define a conductive interconnect feature.
A profile of the deposited metal film, following electroplating, is a measure of the uniformity of the thickness of the metal film deposited at the different locations on the seed layer across the substrate. Usually the profile is taken from the periphery to the center of the seed layer on the substrate. The profile of the deposited metal film thus provides an indication of the uniformity of the deposition rate of the metal film deposited across the seed layer on the substrate. For example, a seed layer location on the substrate that has undergone a relatively heavy deposition of metal film has a thicker profile than a seed layer location on the substrate that has undergone a relatively light deposition of metal film. During the processing that follows the electroplating such as CMP, it is important to determine the profile of the metal film. The profiles can be used to determine the necessary further processing to compensate for varying plating depths and/or undesired cross-sectional shapes of the metal film deposited on the seed layer. For example, assume that a substrate, following processing, has a profile indicating the metal film deposited on the periphery of the substrate seed layer is thicker than the metal film deposited on the center of the substrate seed layer. During the CMP process, more polishing or etching is then performed on the deposited metal film at the periphery of the substrate than the metal film deposited at the center of the substrate to provide a more uniform thickness of metal film following CMP. This selective polishing or etching would be necessary to make the depth of the deposited metal film more consistent across the substrate. Following the CMP process, the profile should indicate a consistent depth of deposited metal film across the seed layer.
Depositing a metal film on an irregularly-shaped object has proven challenging. In certain prior electroplating systems, it has been attempted to control the profile of the deposited metal film on the substrate during the electroplating process. The more consistent the profile thickness of the deposited metal film across the substrate from the center to the periphery, the less polishing or etching has to be performed on the substrate by subsequent processes, such as CMP, to provide a deposited metal film having a uniform depth. Therefore, less wear is provided to the CMP equipment, and less CMP time is necessary if less polishing is necessary.
Electroplating has been applied to other areas than semiconductor processing, such as metal automobile bumpers. Generally during electroplating, the distance of a specific electroplated object point on the seed layer, through the electrolyte solution, to the anode is inversely related to the electric resistance through the electrolyte solution of that electroplated object point. The electric current density applied to the electroplated object point from the anode is a function of the distance between the anode and the electroplated object point. The deposition rate at the electroplated object point is directly related to the electric current density applied to that specific electroplated object point. Therefore, the deposition rate at a specific electroplated object point is inversely related to the distance to the nearest anode point. Providing a desired or consistent electroplated metal deposition rate across the electroplated object is challenging when different points on the electroplated object are located a considerable different distance away from the anode. For example, electroplating large, curved metal objects such as automobile bumpers is challenging since different points on the surface of the automobile bumper are located at considerably different distance away from the anode. If the electric field between the anode and object is not altered during plating, the electroplated object points that are located closest to the anode have the highest deposition rate.
Field-shaping devices, such as shutters and baffles, have been used to compensate non-uniformities in deposited depths rates resulting from different points on the object being electroplated being a different distance from the anode. The field-shaping devices modify the shape of the electric field within an electroplating system, so the electric field will enhance the uniformity of the deposition rate on irregularly-shaped objects. The use of such field-shaping devices as shutters and baffles, however, requires the complexity, maintenance, and expense associated with the insertion and use of another complex mechanical device into the electroplating system. An actuator is typically required to displace the field-shaping device to different positions to adjust for different deposition rates across the surface of the object being electroplated. Additionally, such field-shaping devices as shutters and baffles are often not concentric with the electroplater to enhance the electric field applied to irregularly-shaped objects. The non-concentric shaped field-shaping device therefore often provide other unintended alterations to the electric field within the electrolyte solution that result in other inconsistencies to the deposition rate applied to the seed layer on the substrate.
Another field-shaping process involves electroplating the substrates in one of multiple cells have different shapes or diameters. The different cells therefore each generate an electric field having a different shape. If a substrate shows an undesired profile of metal film deposition on the seed layer after being processed in one electroplating cell, for example, then other similarly sized and configured substrates can be processed in another cell having a different diameter or shape. Different electroplating cells having a different diameters or dimensions produce a different electric field. Changing cells to provide different electric fields requires the expense and time associated with both the use of multiple electroplating cells and changing the cells. There is also the possibility that none of the electroplating cells provide the desired plating characteristics.
Therefore, there remains a need to provide a simple mechanism to control the electric current density of the seed layer on a substrate, from the center to the periphery during electro-chemically deposition of a metal film on the seed layer. The profile of the metal film deposited across the seed layer on the substrate is directly related to the electric current density at the different seed layer locations across the substrate.